1. Field of this Invention
This invention relates to a process for etching and preparing offset plates.
2. Prior Art
German OS NO. 1,571,903 discloses etching multilayered offset printing plates, which consist of a carrier and at least one metal layer on it (preferably a nickel layer), with ferric chloride and copper chloride and a small addition of acid. The metal layer may be etched right through to the carrier, for example, a plastic base. In said German OS, the plastic base is said to be oleophilic without special treatment, so the printing element should need no further treatment.
From the etching of offset printing plates (produced by electroless nickel-plating of a plastic foil and then were coated with a photoresist-layer, exposed and developed), it turned out that the etching solutions described in German OS No. 1,571,903 had the following disadvantages.
When the etching medium concentration is too high, the etching solutions (FeCl.sub.3, CuCl.sub.2) act too aggressively (i.e., etching times are too short). Whenever the concentration of the etching medium is reduced to a point that the etching time are long enough, the etching capacity of the etching solution is insufficient to etch the nickel layer when etching is carried out by hand using only small amounts of solution.
As a result of poor wetting, the nickel layer is etched irregularly.
At the points protected by the photolayer, an underetching takes place when etching with FeCl.sub.3 solution or FeCl.sub.3 /CuCl.sub.2 solutions. Such underetching has the consequence that nickel-dots are partly finer than intended or even the consequence of the complete etching away of fine nickel points.
Whenever a polyester foil (nickel coated according to the electroless plating process), which has been etched photomechanically, is inked directly with printing ink without first treating it in some way, one finds that both the nickel layer as well as the polyester layer take up the ink--the adsorption of the ink by the polyester layer is still capable of improvement, i.e., in the printing machine, the oleophilic polyester parts of the plate show insufficient adsorption of ink and the hydrophilic nickel-parts show adsorption of the ink.
See also: U.S. Pat. No. 3,931,030; U.S. Pat. No. 3,136,670; U.S. Pat. No. 3,293,186; U.S. Pat. No. 3,239,466; U.S. Pat. No. 3,232,802; U.S. Pat. No. 3,672,885; U.S. Pat. No. 3,458,372; and Japanese Pat. No. 7,312,926.